Electroplating baths containing divalent tin and acids such as mineral acids (e.g., sulfuric acid, hydrochloric acid, and hydrofluoric acid), phenol-sulfonic acid, fluoboric acid, and methane sulfonic acid are used in plating tin and tin alloys. A problem is the loss of available divalent tin (Sn2+) due to oxidation of the divalent tin to tetravalent tin (Sn4+). Tetravalent tin accumulates as stannic acid and eventually forms an insoluble sludge in the bath. In addition to removing the amount of divalent tin available for plating, sludge formation also causes equipment fouling and plugging, resulting in an inferior product, along with increased operational costs.
Oxidation of divalent tin occurs at the anode of the electroplating cell, or can result from air introduced into the bath. For example, rapid pumping of plating solution required in the so called “high speed plating” processes result in the inclusion of substantial amounts of oxygen into the bath, which accelerates the oxidation of divalent tin. Accordingly, high-speed tin-plating worsens the sludge problem as compared to other non-high speed tin-plating applications. To prevent this oxidation and the corresponding formation of sludge, divalent tin should remain in solution, and/or be quickly converted back to divalent tin once oxidation has occurred.
Attempts to minimize divalent tin oxidation in plating baths are described, for example, in U.S. Pat. Nos. 5,094,726 and 5,066,367, both to Nobel et al., which are directed to using alkyl sulfonic-acid based tin solutions in combination with antioxidants (also referred to as reducing agents) to prevent a buildup of tetravalent tin. Specifically, Nobel et al. is directed to an electrolyte for electroplating tin or tin-lead alloys comprising a soluble divalent tin compound, a soluble alkyl or alkylol sulfonic acid in an amount sufficient to provide a solution having a pH less than 3, at least one wetting agent, and a hydroxyphenyl compound in an amount sufficient to reduce or prevent the formation of tetravalent tin and tin-oxide sludge. Hydroxyphenyl compounds include pyrocatecol, hydroquinone, resorcinol, phloroglucinol, pyrogallol, 3-amino phenol, or hydroquinone sulfuric acid ester.
However, reducing agents can be an incompatible with wetting agents, sulfonic acids, and other components of tin electroplating baths. These reducing agents react to form insoluble oils and gels, which have a detrimental effect on plating and result in an inferior product by coating heat-transfer surfaces, and/or forming emulsions within the cell. Anti-oxidants (reducing agents) that are compatible with the other components common in plating baths, and which prevent the oxidation of divalent tin and/or stabilize stannous tin to prevent sludge formation are desired.